Many parts of the central nervous system (CNS) exhibit laminar organization, and neuropathological processes generally involve more than one of these multiple cellular layers. Diseases of the CNS frequently include neuronal cell loss, and, because of the absence of endogenous repopulation, effective recovery of function following CNS-related disease is either extremely limited or absent. In particular, the common retinal condition known as age-related macular degeneration (AMD) results from the loss of photoreceptors together with the retinal pigment epithelium (RPE), with additional variable involvement of internuncial (“relay”) neurons of the inner nuclear layer (INL). Restoration of moderate-to-high acuity vision, therefore, requires the functional replacement of some or all of the damaged cellular layers.
Anatomically, retinitis pigmentosa (RP), a family of inherited retinal degenerations, is a continuing decrease in the number of photoreceptor cells which leads to loss of vision. Although the phenotype is similar across most forms of RP, the underlying cellular mechanisms are diverse and can result from various mutations in many genes. Most involve mutations that alter the expression of photoreceptor-cell-specific genes, with mutations in the rhodopsin gene accounting for approximately 10% of these. In other forms of the disease, the regulatory genes of apoptosis are altered (for example, Bax and Pax2). AMD is a clinical diagnosis encompassing a range of degenerative conditions that likely differ in etiology at the molecular level. All cases of AMD share the feature of photoreceptor cell loss within the central retina. However, this common endpoint appears to be a secondary consequence of earlier abnormalities at the level of the RPE, neovascularization, and underlying Bruch's membrane. The latter may relate to difficulties with photoreceptor membrane turnover, which are as yet poorly understood. Additionally, the retinal pigment epithelium is one of the most important cell types in the eye, as it is crucial to the support of the photoreceptor function. It performs several complex tasks, including phagocytosis of shed outer segments of rods and cones, vitamin A metabolism, synthesis of mucoploysacharides involved in the metabolite exchange in the subretinal space, transport of metabolites, regulation of angiogenesis, absorption of light, enhancement of resolution of images, and the regulation of many other functions in the retina through secreted proteins such as proteases and protease inhibitors.
An additional feature present in some cases of AMD is the presence of aberrant blood vessels, which result in a condition known as choroidal neovascularization (CNV). This neovascular (“wet”) form of AMD is particularly destructive and seems to result from a loss of proper regulation of angiogenesis. Breaks in Bruch's membrane as a result of RPE dysfunction allows new vessels from the choroidal circulation access to the subretinal space, where they can physically disrupt outer-segment organization and cause vascular leakage or hemorrhage leading to additional photoreceptor loss.
CNV can be targeted by laser treatment. Thus, laser treatment for the “wet” form of AMD is in general use in the United States. There are often undesirable side effects, however, and therefore patient dissatisfaction with treatment outcome. This is due to the fact that laser burns, if they occur, are associated with photoreceptor death and with absolute, irreparable blindness within the corresponding part of the visual field. In addition, laser treatment does not fix the underlying predisposition towards developing CNV. Indeed, laser burns have been used as a convenient method for induction of CNV in monkeys (Archer and Gardiner, 1981). Macular laser treatments for CNV are used much more sparingly in other countries such as the U.K. There is no generally recognized treatment for the more common “dry” form of AMD, in which there is photoreceptor loss overlying irregular patches of RPE atrophy in the macula and associated extracellular material called drusen.
Since RPE plays an important role in photoreceptor maintenance, and regulation of angiogenesis, various RPE malfunctions in vivo are associated with vision-altering ailments, such as retinitis pigmentosa, RPE detachment, displasia, atrophy, retinopathy, macular dystrophy or degeneration, including age-related macular degeneration, which can result in photoreceptor damage and blindness. Specifically and in addition to AMD, the variety of other degenerative conditions affecting the macula include, but are not limited to, cone dystrophy, cone-rod dystrophy, malattia leventinese, Doyne honeycomb dystrophy, Sorsby's dystrophy, Stargardt disease, pattern/butterfly dystrophies, Best vitelliform dystrophy, North Carolina dystrophy, central areolar choroidal dystrophy, angioid streaks, and toxic maculopathies.
General retinal diseases that can secondarily affect the macula include retinal detachment, pathologic myopia, retinitis pigmentosa, diabetic retinopathy, CMV retinitis, occlusive retinal vascular disease, retinopathy of prematurity (ROP), choroidal rupture, ocular histoplasmosis syndrome (POHS), toxoplasmosis, and Leber's congenital amaurosis. None of the above lists is exhaustive.
All of the above conditions involve loss of photoreceptors and, therefore, treatment options are few and insufficient.
Because of its wound healing abilities, RPE has been extensively studied in application to transplantation therapy. In 2002, one year into the trial, patients were showing a 30-50% improvement. It has been shown in several animal models and in humans (Gouras et al., 2002, Stanga et al., 2002, Binder et al., 2002, Schraermeyer et al., 2001, reviewed by Lund et al., 2001) that RPE transplantation has a good potential of vision restoration. However, even in an immune-privileged site such as the eye, there is a problem with graft rejection, hindering the progress of this approach if allogenic transplantation is used. Although new photoreceptors (PRCs) have been introduced experimentally by transplantation, grafted PRCs show a marked reluctance to link up with surviving neurons of the host retina. Reliance on RPE cells derived from fetal tissue is another problem, as these cells have shown a very low proliferative potential. Emory University researchers performed a trial where they cultured RPE cells from a human eye donor in vitro and transplanted them into six patients with advanced Parkinson's Disease. Although a 30-50% decrease in symptoms was found one year after transplantation, there is a shortage of eye donors, this is not yet FDA approved, and there would still exist a need beyond what could be met by donated eye tissue.
Thus far, therapies using ectopic RPE cells have been shown to behave like fibroblasts and have been associated with a number of destructive retinal complications including axonal loss (Villegas-Perez, et al, 1998) and proliferative vitreoretinopathy (PVR) with retinal detachment (Cleary and Ryan, 1979). RPE delivered as a loose sheet tends to scroll up. This results in poor effective coverage of photoreceptors as well as a multilayered RPE with incorrect polarity, possibly resulting in cyst formation or macular edema.
Delivery of neural retinal grafts to the subretinal (submacular) space of the diseased human eye has been described in Kaplan et al. (1997), Humayun et al. (2000), and del Cerro et al. (2000). A serious problem exists in that the neural retinal grafts typically do not functionally integrate with the host retina. In addition, the absence of an intact RPE monolayer means that RPE dysfunction or disruption of Bruch's membrane has not been rectified. Both are fundamental antecedents of visual loss.
Thus, there exists no effective means for reconstituting RPE in any of the current therapies and there remain deficiencies in each, particularly the essential problem of a functional disconnection between the graft and the host retina. Therefore there exists the need for an improved retinal therapy.